Observations of Mud‐Induced Periodic Stratification in a Hyperturbid Estuary. Issue 11 (5th June 2018)

• Record Type: Journal Article
• Authors: Becker, Marius; Maushake, Christian; Winter, Christian
• Is Part Of: Geophysical research letters. Volume 45:Issue 11(2018)
• Page Start: 5461
• Page End: 5469
• Publication Date: 2018-06-05
• Subjects: estuarine dynamics -- periodic stratification -- entrainment -- fluid mud -- tidal asymmetry -- density effects; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL077966
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: This study presents new observations of fluid mud transport and of the interaction between mud‐induced stratification and the flow. Data collected in a hyperturbid estuarine tidal channel reveal details of the intratidal entrainment asymmetry, characterized by quasi‐instantaneous entrainment and upstream pumping of mud during flood, and a gradual reduction of layer thickness by shear dispersion during ebb. Rapid restratification early during the flood phase restores the predominant two‐layer structure and delimits the transport period, which is then significantly shorter than the overall flood duration. The hydraulic cross section is reduced, causing an increase of the salinity intrusion into the estuary at the end of the flood. The fluid mud layer occupies on average 40% of the water depth, and stratification exceeds that of highly stratified salt wedge estuaries. These data show how mud‐induced periodic stratification influences flow structure and sediment transport and thereby contribute to the understanding of the dynamics of hyperturbid estuaries. Plain Language Summary: Deepening of navigational channels often leads to an increase of turbidity in estuaries. This increase may be extreme, with negative side effects for the environment, which are intensively discussed at present. While mud accumulates, it does not consolidate but is periodically mixed by tidal currents. This stage, called fluid mud, in turn changes the flow, in particular the vertical profile of flow velocities. This feedback is known in theory, but little information has been collected in the field. Long‐term transport of mud in turbid estuaries is analyzed using models, but it is unclear if model results reflect the actual situation in nature. Due to the described effect of mud on the flow, detailed measurements are required, such as they are presented in this study. These data reveal several effects of mud on the flow, which have not been shown in sufficient detail before. An important aspect is the strong difference in mixing of mud between the ebb and the flood phase, which leads to a corresponding difference in mud transport. Any long‐term change, in import or export of mud, depends on this balance. Therefore, results presented in this study significantly contribute to the understanding of mud transport in turbid estuaries. Key Points: Forced by an asymmetric tide, the flow structure in a hyperturbid tidal channel is controlled by mud‐induced periodic stratification Restratification during flood leads to vertical decoupling, counterdirected flow, and the development of an inverse salinity profile Intratidal transport of mud depends on the entrainment asymmetry, inducing upstream pumping during flood and shear dispersion during ebb